Ah yes, the iMac "G9"... (?)
the m5521 refers to what I call a second generation iMac G3. (The G3 refers to the G3 processor)
It might have 2 sticks of 512MB RAM, for a maximum total of 1 GB.
The hard drive can be "upgraded" to a maximum size of 120 GB, and you can go to OS X 10.4.11, no newer...
You can find out lots more with the MacTracker app, or one of the various sites that shows history of older models from Apple, Inc.
What else do you want to know?